1. Technical Field
The present invention relates to optical semiconductor elements that emit laser light and methods for manufacturing the same.
2. Related Art
A surface-emitting type semiconductor laser is a type of optical elements that emit laser light. The surface-emitting type semiconductor laser is provided with a resonator formed in a direction orthogonal to a surface of the substrate, and emits laser light from the substrate surface. Compared to conventional edge-emitting type semiconductor lasers that use parallel cleavage surfaces of a substrate as a resonator, the surface-emitting type semiconductor laser has various favorable characteristics. For example, surface-emitting type semiconductor lasers are suitable for mass-production, capable of direct modulation, capable of operation with low threshold levels, capable of oscillation in a single longitudinal mode, and a two-dimensional laser array structure can be readily formed with surface-emitting type semiconductor lasers.
A surface-emitting type semiconductor laser has a smaller device volume compared to an ordinary edge-emitting type semiconductor laser, such that the electrostatic breakdown voltage of the device itself is low. When the electrostatic breakdown voltage is low, the device may be damaged by static electricity caused by a machine or an operator while the device is mounted on a substrate or a pedestal. For this reason, a variety of measures are implemented in a device mounting process to remove static electricity. To remove static electricity from operators, for example, the operators wear working dresses made of antistatic fabric during work, humidity of the work environment is controlled, and the work environment is always placed in an electrically neutralized state by using ionizers. However, these measures have limitations, and the possibility of destruction of devices having an electrostatic breakdown voltage of about 200V or lower during mounting process becomes higher. In this respect, for example, Japanese Laid-open Patent Application JP-A-2004-6548 describes a semiconductor laser with an improved electrostatic breakdown voltage.
Furthermore, surface-emitting type semiconductor lasers have characteristics in which their optical output changes according to the ambient temperature. In this respect, Japanese Laid-open Patent Applications JP-A-2005-33106 and JP-A2005-197513 describe semiconductor elements in which a light-receiving element such as a photodiode is provided on a surface-emitting type semiconductor laser, a portion of laser light emitted from the surface-emitting type semiconductor laser is detected for monitoring by the photodetecting element, and outputs of the surface-emitting type semiconductor laser are controlled based on the monitored results.
It is noted that surface-emitting type semiconductor lasers have breakdown voltage characteristics in which their breakdown voltage is relatively high against a forward bias, which is about 500V or higher, but relatively low against a reverse bias, which is 300 V or lower. For this reason, it is effective to implement measures against reverse bias for improving the electrostatic breakdown voltage of surface-emitting type semiconductor lasers. For example, according to one of such measures, a surface-emitting type semiconductor laser may be provided with a diode having inverse characteristics, which is connected in parallel with the surface-emitting type semiconductor laser as an electrostatic breakdown element. However, if the electrostatic breakdown element is provided within a single chip separately from the surface-emitting type semiconductor laser, the design margin becomes small, which makes it difficult to design photo masks that are to be used for manufacturing the optical semiconductor element.
Moreover, surface-emitting type semiconductor lasers include an eye-safe measure to secure the safety of the person's retinas. More concretely, when an optical sub-assembly (OSA) such as a transmitter optical sub-assembly (TOSA) is equipped with a surface-emitting type semiconductor laser, a reflection film may be formed at a laser light emission window provided on the TOSA as a measure to adjust the transmittivity of laser light to thereby prevent emission of excessive laser light. However, if the laser light emitted from the surface-emitting type semiconductor laser is reflected by the reflection film and enters the surface-emitting type semiconductor laser, noise could be generated.